Electrostatographic printing operates according to the principles and embodiments of non-impact printing as described, eg, in "Principles of Non-Impact Printing" by Jerome L Johnson (1986)--Palatino Press--Irvine Calif., 92715 USA).
Electrostatographic printing includes electrographic printing in which an electrostatic charge is deposited image-wise on a dielectric recording member as well as electrophotographic printing in which an overall electrostatically charged photoconductive dielectric recording member is image-wise exposed to conductivity increasing radiation producing thereby a "direct" or "reversal" toner-developable charge pattern on said recording member. Magnetic brush development is suited for "direct" as well as "reversal" development. "Direct" development is a positive-positive development, and is particularly useful for reproducing pictures rather than text. "Reversal" development is of interest in or when from a negative original a positive reproduction has to be made or vice-versa, or when the exposure derives from an image in digital electrical signal form, wherein the electrical signals modulate a laser beam or the light output of light-emitting diodes (LEDs). It is advantageous with respect to a reduced load of the electric signal modulated light source (laser or LEDs) to record graphic information (eg printed text) in such a way that the light information corresponds with the graphic characters so that by "reversal" development in the exposed area of a photoconductive recording layer, toner can be deposited to produce a positive reproduction of the electronically stored original. In high speed electrostatographic printing the exposure derives practically always from electronically stored, ie computer stored, information.
As used herein, the term "electrostatographic" also includes the direct image-wise application of electrostatic charges on an insulating support, for example by ionography.
The toner image obtained on a repeatedly used electrostatographic dielectric recording member is transferred onto a printing stock material, usually paper, in the form of a web whereon the toner image is fixed, whereupon the web is usually cut into sheets containing the desired print frame.
The printing device according to the invention is particularly concerned with the image formation, developing, transfer and fixing of multi-colour images on a travelling web, whereby utmost care has to be taken that the selection images, which make up said multi-colour image overlap each other according to very close tolerances. Mis-registering or too large tolerances could give rise to small colour stripes becoming visible or to the creation of a moire-pattern, whereby a low frequency interference pattern originates on or in the final multi-colour image.
The problem of transferring selection images in register towards a travelling web is well known in the art.
In graphic arts, with gravure, offset and flexography being the dominant printing techniques, and wherein either a printing plate is positioned on a printing cylinder or the printing cylinder itself has an image pattern etched into its surface, use is made of so-called register marks which are printed on the image-receiving web to be printed upon. In the course of printing those register marks are generally optically detected and the detected signal is sent to a circuit controlling a servomotor which is capable of acting upon a differential gear system forming part of the printing cylinder and by means of which small relative movements of the printing plate relative to the circumferential surface of the printing roller may be made for correction of register.
In British patent application GB-A-2195856 (Matsushita), there is described a sensing device for sensing marks formed on a moving web in order to ensure correct registration. In U.S. Pat. No. 4,912,491 (Hoshino et al. assigned to Canon KK) there is described another apparatus which utilises registration marks formed on the image transferring medium. The registration mark is formed on a transparent part of the medium. Another disclosure of an apparatus utilising registration marks on the image-receiving member is U.S. Pat. No. 5,160,946 (Hwang assigned to Xerox Corporation).
A disadvantage of the use of sensing marks on the moving web is that their presence on the final product is unacceptable, which means that steps have to be taken for their removal, for example by deletion or by cutting of the web.